The present invention relates to magnetic bubble memory devices using ion-implanted tracks, and particularly to an improvement in prevention of simultaneous generation of a plurality of bubbles in a generator using the ion-implanted track and a hairpin conductor pattern for a magnetic bubble memory chip.
As to a magnetic bubble generator composed of the ion-implanted track and the conductor pattern, a discussion is made, for instance, in The Bell System Technical Journal, Vol. 59, No. 2 (1980), pp. 229-257. As is shown in FIG. 2, an insulating layer omitted in the figure of Si0.sub.2 or the like is superposed on an ion-implanted track 1, and a conductor pattern 5 of a hairpin type is put thereon. When the temperature of a chip having a bubble magnetic film, an insulating film and the conductor pattern (hereinafter simply called a chip) is 85.degree. C., a current pulse with the amplitude of about 120 mA is made to flow through the hairpin conductor 5 to generate inside the hairpin gap 4 surrounded by said hairpin conductor 5, a magnetic field being reverse in direction to a bias magnetic field. A magnetic bubble is generated by inverting the magnetization of the bubble magnetic film locally by said magnetic field. The width of the current pulse required for generating the magnetic bubble is about 100 to 200 ns.
In FIG. 2, numeral 2 denotes an ion-implanted region, 3 a non-implanted region and 4 a cusp of the ion-implanted track 1. The generator using the ion-implanted track 1 and the hairpin conductor 5 according to the above-described prior art has a problem that two or more bubbles are generated simultaneously. When the current pulse is supplied through the hairpin conductor 5 shown in FIG. 2, a magnetic domain wherein magnetization is inverted is nucleated and said magnetic domain is stretched along the inside of the hairpin conductor 5. When the current pulse falls, the stretched magnetic domain begins to shrink and returns to a circular magnetic domain within several hundreds of nanoseconds. A part of said stretched magnetic domain is occasionally separated, in said bubble stretch and shrink operation, into two or more magnetic bubbles. The same phenomemon is observed in a generator composed of a Permalloy (Ni-Fe system high permeability alloy) track with a conductor pattern, and in this relation, it is described in IEEE, Trans. Magnetics, MAG-15, No. 6 (1979), pp. 1715-1717. The said erroneous operation of the generator using Permalloy tracks can be prevented by using a current pulse with a fall time of 100 to 400 ns. This method is effective for a magnetic bubble having a diameter of 1.5 .mu.m, but it is not sufficiently effective when a magnetic bubble has a diameter smaller than 1 .mu.m.
The factors relating to erroneous bubble generation, for diameters as large as 2 .mu.m, are described in IEEE, Trans. Magnetics, MAG-16 (198), pp. 1389-1395. According to the description, a magnetic domain 10 stretches as shown in FIG. 5A immediately after the bubble is generated. A part of the domain stretching over the outside of a gap part of a hairpin conductor 5 is separated to cause the generation of an extra bubble. For a counter-measure, one effective method has been to prolong the shrinking time by making the fall time of the current pulse longer.
However, the result of studies by the present inventors has recently discovered that only making the fall time longer cannot prevent said erroneous generator operation for a generator with conductor pattern width of 3 to 4 .mu.m and gap width of 1 .mu.m when the bubble diameter is smaller than 1 .mu.m. This is due to the reason discussed hereinbelow. In the case when the bubble diameter is small, the bubble generated in the gap of the hairpin conductor stretches not only in the direction of the hairpin, but also in the perpendicular direction. The stretched domain form is as shown in FIG. 5B. Consequently, the wave-shaped magnetic domain wall (boundary of the magnetic domain 10) is formed just after the bubble is generated by the current pulse supply. The error that the magnetic domain 10 is divided into two or more easily occurs after the current pulse falls and said error cannot be prevented, accordingly, even by making the fall time of a pulse current as long as 100 to 400 ns. In other words, such wave-shaped magnetic domain wall as mentioned above is formed and two or more bubbles are generated when the current pulse is supplied through the hairpin conductor, if a region wherein a magnetic pulse being reversed in direction to a bias field is generated has a width three times larger than the bubble diameter.
According to the studies of the present inventors, said problem can be solved by making the pattern width of the hairpin conductor and the width of the gap smaller proportional to the diameter of the bubble. In other words, it can be solved by narrowing the width of the region wherein the magnetic pulse is impressed. However, when the pattern width of the conductor is smaller, the conductor pattern migration due to the large current density results in an insufficient lifetime of the conductor pattern. It is also difficult to delineate the hairpin conductor pattern with the small width and the small gap.